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Abstract

We present a new type of hydrogel photonic crystal with a stop band that can be rapidly modulated across the entire visible spectrum. We make these materials by using a high-molecular-weight polymer to induce a depletion attraction between polystyrene-poly(N-isopropylacrylamide-co-bisacrylamide-co-acrylic acid) core-shell particles. The resulting crystals display a stop band at visible wavelengths that can be tuned with temperature at a rate of 60 nm/s, nearly three orders of magnitude faster than previous photonic-crystal hydrogels. Above a critical concentration of depleting agent, the crystals do not melt even at 40 degrees Celsius. As a result, the stop band can be modulated continuously from red (650 nm) to blue (450 nm), with nearly constant reflectivity throughout the visible spectrum. The unusual thermal stability is due to the polymer used as the depleting agent, which is too large to enter the hydrogel mesh and therefore induces a large osmotic pressure that holds the particles together. The fast response rate is due to the collective diffusion coefficient of our hydrogel shells, which is more than three orders of magnitude larger than that of conventional bulk hydrogels. Finally, the constant reflectivity from red (650 nm) to blue (450 nm) is due to the core-shell design of the particles, whose scattering is dominated by the polystyrene cores and not the hydrogel. These findings provide new insights into the design of responsive photonic crystals for display applications and tunable lasers.

Fig. 2 PS/PNiPAm-BIS-AAc hydrogel photonic crystals display a continuous shift in the position of their stop band without melting. a) Reflectance spectra of photonic crystals taken during step-wise heating from 20°C to 40°C with a step size of 1°C. b) Peak position (λmax) as a function of temperature during heating (black circles) and cooling (white circles). We equilibrate the samples for 2 min at each temperature before measuring the spectrum. c) A series of optical micrographs of the photonic crystals taken during heating shows the uniform change in structural color upon heating.

Fig. 3 PS/PNiPAm-BIS-AAc photonic crystals respond quickly to changes in temperature. a, b) Peak wavelength λmax (black circles) and temperature (red circles) as a function of time during a) heating and b) cooling. After a few-second delay between the photonic-crystal response and the temperature change, Δt, the stop-band position shifts across the entire visible range with a time constant of 3 sec (heating) or 7.5 sec (cooling). c) The stop-band position λmax as a function of temperature during cyclic swelling and deswelling of the crystals. The temperature is monitored by a thermistor, which is embedded in a dab of thermal paste confined between the sample and heating surface.

Fig. 4 The concentration of non-adsorbing polymer is critical to the stability of self-assembled photonic crystals. a) Normalized full-width at half maximum, Δλ/λmax, of colloidal crystals prepared with different polymer concentrations as a function of temperature: 2.9 g/L (triangles), 4.6 g/L (squares), and 5.7 g/L (circles). We consider the crystals to be melted when Δλ/λmax exceeds 0.2. b) Equilibrium phase diagram in polymer concentration-colloid volume fraction phase space. The phase boundary is computed according to methods in Ref. [30]. The effective size of the polymer depletant is taken to be 62.5 nm, consistent with dynamic light scattering measurements. Lumped contributions due to solid entropy and van der Waals energy are treated as a free parameter, which is constrained by our data at three different polymer concentrations. Points show experimental particle volume fractions for the experimental conditions in a). Open symbols indicate the observed fluid phase, whereas filled symbols indicate the observed crystal phase.

Fig. 5 a) Reflectance spectra of photonic crystals taken during step-wise cooling from 40°C to 20°C with a step size of −1°C. b) A series of optical micrographs of the photonic crystals taken during cooling.

Fig. 6 The volume of a macroscopic hydrogel of poly(NiPAm-co-BIS-AAc) decreases rapidly with increasing temperature inside a glass vial (photos). Open circles show the normalized radius R/R0 of the gel cylinder as a function of time, where R0 is the initial radius; closed circles show the temperature as a function of time. Red curves show exponential fits.